The present invention relates to process variable transmitters used to measure temperature in process control and monitoring systems. More specifically, the present invention relates to monitoring a noise level within a measurement circuit as the measurement of the process variable is being digitized.
Process variable transmitters are used to measure process parameters in a process control or monitoring system. Microprocessor-based transmitters often include a sensor, an analog-to-digital converter for converting an output from the sensor into a digital form, a microprocessor for compensating the digitized output, and an output circuit for transmitting the compensated output. Currently, this transmission is normally done over a process control loop, such as 4-20 milliamp control loop, or wirelessly.
One exemplary parameter that is measured by such a system is temperature, which is sensed by measuring the resistance of a resistive temperature device (RTD), which is also sometimes called a platinum resistance thermometer or (PRT) or the voltage output by a thermocouple. Of course, these types of temperature sensors are only exemplary and others can be used as well. Similarly, temperature is only one exemplary process variable and a wide variety of other process control parameters can be measured as well, such as pressure, flow, pH, etc. Therefore, while the present discussion proceeds with respect to a temperature sensor, it will be appreciated that the discussion could just as easily proceed with respect to other sensors.
There are a number of connection points, in conventional measurement circuitry, between a temperature sensor and a measurement transmitter that can fail or become degraded. In many temperature measurement applications, it can be very important for the user to understand when the temperature measurement is degraded for any reason, before using the measurement in a control strategy. Degraded conditions can be due to a variety of different reasons, including lead/sensor breakage, excessive line resistance, corroded or loose connections, etc.
A number of diagnostics have been implemented in temperature transmitters in order to evaluate static conditions, such as lead resistance, excessive DC voltage, or sensor breakage. However, any of these static conditions can become transient, dynamic, or non-persistent states that occur during the processing of the measurement. This can lead to a number of sources of measurement inaccuracy.
For instance, during the digitizing process, the voltage input to a measurement analog-to-digital (A/D) converter is averaged, and no information about the noise over the digitization process is provided. Asymmetric noise can cause measurement inaccuracy in the output provided to a control system.
In addition, noise can currently be evaluated from sample-to-sample within a transmitter at the update rate of the transmitter, but in some cases, this can be too slow. Measurement noise that occurs at a higher frequency than the update rate can therefore cause inaccuracy as well.
In addition, excessive line resistance or measurement capacitance (either on the sensor line or internal to the transmitter) can affect the settling time of the measurement circuit. If the settling time is not set long enough, measurement inaccuracies beyond the transmitter's accuracy specification can occur. However, increasing the settling time directly reduces the measurement update rate. Therefore, there can be a tradeoff between the measurement update rate and measurement inaccuracies that occur due to a settling time that is too short.